Recently, the need to develop next-generation clean energy is increasing in importance due to serious environmental pollution problems and the exhaustion of fossil energy. Especially, solar cells, which are used to directly convert solar energy into electric energy, are expected to become an energy source able to solve the energy problems of the future because they generate less pollution, utilize unlimited solar resources and have a semi-permanent lifespan.
Solar cells are classified into a variety of types depending on the material for an absorber layer. Currently, the most commonly used is a Si solar cell using Si. However, as the price of Si has drastically increased attributable to the recent Si supply shortage, thin-film solar cells are receiving attention. Thin-film solar cells are thin and enable smaller amounts of materials to be consumed, and are also light and have a wider range of utilization. Thorough research is ongoing into using amorphous Si and CdTe, CIS (CuInSe2) or CIGS (CuIn1-xGaxSe2) as materials of such thin-film solar cells.
A CIS or CIGS thin film corresponds to a Group I-III-IV compound semiconductor, and exhibits the highest conversion efficiency (about 19.9%) among thin-film solar cells which have been experimentally produced. In particular, this thin film may be manufactured to a thickness of 10 μm or less and is stable even upon extended use, and is thereby expected to replace Si to thus fabricate inexpensive high-efficiency solar cells.
Furthermore, the CIS thin film is a direct transition type semiconductor and may thus be provided in the form of a thin film, and has a band gap of 1.04 eV that is comparatively adapted for light conversion, and the light absorption coefficient thereof is the greatest amongst the known solar cell materials. The CIGS thin film has been developed by replacing a portion of In with Ga or replacing Se with S to improve the low open-circuit voltage of the CIS thin film. However, the CIS or CIGS thin film has comparatively high production costs because of the use of expensive In and Ga elements, and the band gap thereof is slightly low.
In order to increase the efficiency of solar cells and achieve cost savings, novel materials and manufacturing able to further increase a band gap and to use inexpensive elements have to be devised.
As part of the recent efforts for developing novel materials to replace the In element with inexpensive elements, the preparation of a Cu2ZnSn(S,Se)4 thin film (hereinafter referred to as “CZTS-based thin film”) wherein In is replaced with inexpensive Zn and Sn is under active study.
However, because the CZTS-based thin film has lower efficiency than that of the CIS or CIGS thin film, extensive and intensive research into increasing the efficiency thereof is being carried out.
Meanwhile, because the conduction band of the CIGS thin film is determined by a bonding relation of Ga and In, the band gap may be changed by way of changing the Ga/(In+Ga) ratio (“The effect of Ga-grading in CIGS thin film solar cells”, Thin Solid Films, Volumes 480-481, 1 Jun. 2005, Pages 520-525), and FIG. 6 is a graph illustrating changes in band gap depending on changes in the Ga proportion in the CIGS thin film.
Moreover, the band gap of CIGS may be controlled by adjusting the Ga proportion, and thereby the Ga and In proportions are changed in the course of preparation of the CIGS thin film, thus increasing the efficiency of the CIGS thin-film solar cell using a double grading structure in which a double band gap slope is imparted in the CIGS thin film.
FIG. 7 schematically illustrates the case where a double band gap slope is formed in a CIGS thin film (“High efficiency graded bandgap thin-film polycrystalline Cu(In,Ga)Se2-based solar cells”, Solar Energy Materials and Solar Cells 41/42 (1996) 231-246).
When the band gap of the front side of the CIGS thin film is higher than that of the center thereof, open-circuit voltage may increase and recombination may be reduced. Also, when the band gap of the back side of the CIGS thin film is higher than that of the center thereof, electron mobility may increase.
However, the CZTS thin film is disadvantageous because the band gap of the CZTS-based thin film cannot be changed by way of changing the Zn/Sn ratio, making it impossible to attain improvements in solar cell efficiency through the double grading structure.